Plug-in connectors have long been used for disconnecting and reconnecting groups of wires, for interrupting and then reconnecting two pieces of electrical apparatus. The connections commonly include energizing circuits, control circuits and monitoring circuits, extending between what may be called "a main electrical unit" and a secondary electrical unit. In an example, the main electrical unit may be a motor control center, and the secondary unit may be the controlled and monitored unit.
In normal operation, the terminal blocks are in their plugged-in condition. The main electrical unit commonly includes a main switch or a circuit breaker or fuses, for turning on power to the secondary unit and for interrupting the power when a fault occurs. There are times when the plug-in terminal blocks are pulled apart, as for safely working on the secondary equipment.
Plug-in terminal blocks contain mating or companion contacts that commonly develop large amounts of friction where they engage and disengage each other. Cam levers are often used for making it easy to plug-in and extract one plug-in terminal-block from a companion terminal block. Resilient detents are occasionally used for increasing the force needed to unplug one connector from the other, to reduce the risk of accidental disconnection.